1. Field of the Invention
This invention relates to seismic source devices and more particularly to a shuttle controlled seismic source device which allows only a portion of the compressed air in the main chamber of the device to be discharged into the surrounding environment.
2. Description of the Prior Art
In marine seismic exploration, a source of acoustic energy is released into the water every few seconds to obtain appropriate acoustic waves that propagate into the earth's surface. These waves are reflected at interfaces of the subsurface formations and propagate back to instruments where transducers convert the acoustic waves to electronic signals which are recorded and later processed into a record section for interpretation of the subsurface formations.
In recent times, the major marine seismic energy source has been the air gun. These air guns release high pressure air, typically 2,000 psig to 6,000 psig into the water to create the desired acoustic wave.
Many conventional air guns comprise an annular housing that contains means for discharging compressed air through exhaust ports in the housing. Compressed air is stored within the housing in a main chamber. The only moving component (except for the solenoid triggering device) is a shuttle, which when raised, permits air to escape from the main chamber through the ports in the main housing into the surrounding water. The size of the gun is determined by the main chamber volume selected. By having a constant source of compressed air through an inlet passage in the housing, the upper chamber containing the shuttle is filled and forces the shuttle into a sealed position, closing off all exhaust ports from the main chamber. By using a solenoid valve to allow air flow beneath the shuttle face and cause an unequal pressure on the shuttle, the shuttle is accelerated in the upward direction, exposing the chamber exhaust ports and allowing compressed air to escape into the surrounding water. When the shuttle is in the "down," or pre-fire position, the air gun is charged and ready for firing.
Air guns are typically deployed underwater in an array. When fired, the air guns nearly simultaneously release compressed air with a velocity at or near Mach 1. The discharging air from each gun forms a substantially spherical bubble having an air/water interface that translates radially outward from the gun at some initial acceleration. The rapid expansion of the air/water interface causes the desired compression wave. The point in time when the acceleration of the air/water interface declines to approximately zero corresponds to the maximum acoustic pressure generated by each individual gun.
At some instant in time after the maximum acoustic pressure is reached, the bubble generated by each gun begins to collapse under the pressure of the surrounding water. The collapsing bubble does not collapse instantaneously, but rather tends to collapse and reexpand cyclically with a frequency of oscillation that is peculiar to the particular gun. The oscillatory collapse of the bubble generates a series of secondary acoustic pulses, which, if not filtered, obscure the behavior of the acoustic waves from the first bubbles.
The sizes of the individual guns are tailored to the sizes of the other guns in the array so that the amplitudes of the initial pulses from each gun constructively interfere or add, while the undesirable secondary acoustic pulses from the oscillatory collapse of the bubbles destructively interfere.
Experimentation has shown that it is more difficult to tailor the array of air guns to use destructive interference to successfully nullify secondary acoustic pulses from relatively larger (as opposed to smaller) bubbles from the air guns. Thus, it is desirable to have the air guns produce as small bubbles as possible.
When fired, the typical air gun allows 80% to 90% of the air in the main chamber to be exhausted into the water. However, as noted above, during the firing sequence of this type of air gun, the acoustic pressure generated by the air discharging from the main chamber will rapidly increase to a maximum and then begin to tail off, shortly after the air begins to discharge from the main chamber and well before the main chamber is completely discharged. Thus, any air discharged from the main chamber after the maximum acoustic pressure is reached is wasted, and will form another bubble that will create an undesired pulse.
Large losses of air mass from the main chamber result in two undesirable effects: (1) the requirement for unnecessarily large shipboard compressor equipment; and (2) the production of large bubbles that may produce secondary pulses that prove difficult to filter. Large losses of air volume from the main chamber require more time and more air, and thus larger shipboard compressor equipment, to recharge the air gun between firing cycles.
The prior art contains a number of devices designed to control the amount of compressed air lost during firing. These previous designs have utilized a dual shuttle arrangement wherein the first shuttle is configured to enable compressed air to escape into the surrounding water, and the second shuttle is configured to close the opening through which the compressed air is discharged to minimize the amount of compressed air lost during each firing cycle. In these devices, typically the second shuttle is activated by the air discharge flow resulting from the action of the first shuttle. The action of the discharging air flowing around the second shuttle causes it to close.
Due to the unavoidable variations in the air flow around the second shuttle as a result of surface imperfections on the second shuttle, irregular kinetic friction between the second shuttle and the housing, and the non-linear behavior of air flowing at very high Reynolds numbers, the closing time for the second shuttle may vary greatly from one firing cycle to the next. The result is wide variation in the air bubble size that is discharged from the gun and the resulting period of oscillation. Filtering out such variations by use of destructive interference or other methods has proven difficult.
The present invention is directed to overcoming or minimizing one or more of the problems discussed above.